This invention relates to a semiconductor device comprising an insulated gate field effect transistor.
In particular, this invention relates to a semiconductor device comprising an insulated gate field effect device having a semiconductor body comprising a first region of one conductivity type, a second region of the opposite conductivity type forming a first pn junction with the first region, a third region of the one conductivity type forming a second pn junction with the second region and being separated from the first region by the second region, at least one injector region for injecting charge carriers of the opposite conductivity type into the first region, an insulated gate, a conduction channel area within the second region and adjoining the insulated gate, which conduction channel area is gateable by the insulated gate between a first state in which a conduction channel of the one conductivity type provides a conductive path for the flow of charge carriers of the one conductivity type between the first and third regions and a second state in which the conduction channel is removed, the device being such that initiation of thyristor action by the transistors formed by the first, second and third regions and the injector, first and second regions is inhibited.
Such an insulated gate field effect device is generally now known as an insulated gate bipolar transistor (IGBT) or sometimes an insulated gate transistor (IGT). This and other types of combined MOS and bipolar devices are described in a paper entitled "Evolution of MOS-Bipolar Power Semiconductor Technology" by B. Jayant Baliga published in the Proceedings of the IEEE Volume No. 76, April 1988 at pages 409 to 418.
An insulated gate bipolar transistor (IGBT) differs from a conventional insulated gate field effect transistor in that, in the case of an enhancement type device, in the on-state in addition to the current of the one conductivity type provided by carriers of the one conductivity type flowing along the inversion channel induced by the first voltage applied to the insulated gate structure, a current of the opposite conductivity type is provided by the injection of opposite conductivity type carriers into the first region by the injector region. This injection of opposite conductivity type carriers into the first region reduces the on-resistance of the device in comparison to that of a similar construction conventional insulated gate field effect transistor or MOSFET. As indicated by Baliga, the IGBT exhibits the desirable features of a MOSFET, namely voltage controlled operation and high output impedance as well as the desirable features of bipolar devices namely high forward conduction density albeit at the price of a reduced switching speed compared to conventional MOSFETS.
The IGBT has advantages over the earlier MOS-gated thyristor in that, unlike the MOS-gated thyristor, the IGBT can still be easily controlled by a voltage applied to the insulated gate structure after the device has been turned on. In an attempt to make the MOS-gated thyristor more controllable, the MOS-controlled thyristor (MCT) was devised in which, as described by Baliga, a second insulated gate field effect device structure or MOS is used to turn off the thyristor.
Compared to the MCT, the IGBT has been found to have an improved controllable current capability and safe operating area (SOA). However, the on-resistance of the IGBT is in comparison higher because of the potential drop along the conduction channel and because of less than effective conductivity modulation of the first or drift region. Moreover, the necessity to avoid breakdown by punch through when the IGBT is operating in a reverse voltage blocking mode limits the degree to which the conduction channel length and the dopant concentration of the second region can be reduced. Accordingly, the on-state or conducting performance of the IGBT has generally been found to be inferior to that of the MCT.
As indicated above the MCT requires a first turn-on and a second separate turn-off insulated gate field effect device or MOS structure. In order to achieve turn-off of the thyristor, the second MOS structure is rendered conducting by applying the appropriate voltage to its insulated gate so that charge carriers have an alternative path to the cathode electrode by-passing the cathode junction. However, the thus-diverted current tends to forward bias the cathode junction and so to maintain thyristor action. Accordingly in the MCT it is necessary to make the resistance of the turn-off MOS conduction channel so low that the voltage across it is less than the potential barrier (0.7 V) of the cathode junction and is thus insufficient to maintain thyristor action. Although theoretically this condition can be achieved, the turn-off process of the MCT has been found to be difficult to control for high current densities.